The goal of this predoctoral fellowship is to allow the applicant to further develop basic research skills necessary for an independent career investigating the neural control of movement. The goal of the proposed research is to better understand the flexibility of firing rate interactions among primary motor cortex (M1) neurons, and how this flexibility may be constrained by three specific factors--the spatial proximity and motor output similarity of M1 neurons and the motor experience of the subject. Experiments will address two specific aims: 1) how does the motor output similarity or spatial proximity of M1 neurons affect the flexibility of their firing rate interactions, and 2) how does motor training affect the flexibility of firing rate interactions between M1 neurons? To address these questions, microelectrode arrays will be implanted in the upper extremity region of M1 and bipolar electrodes in the muscles of the contralateral arm. Each experimental session with consist of four epochs. The first epoch, a two-dimensional center-out task, will be used to identify the movement/muscle outputs of all recorded M1 neurons. The second epoch, a two-dimensional brain-computer interface (BCI) task, will probe the joint firing rate space for two selected M1 neurons to assess the degree of flexibility of their firing rate interactions. The third epoch, the center-out task, will consist of repeated movements in only one direction, serving as a massed-practice motor training exercise. The fourth and final epoch, the BCI task, will re-assess the flexibility of firing rate interactions between the selected pair of M1 neurons. To address aim 1, neurons will be paired in the BCI task according to the similarity of their motor outputs and their spatial proximity. To address aim 2, the effect of massed-practice motor training on the flexibility of firing rate interactions between M1 neurons will be investigated. The results of the proposed experiments will provide a better understanding of what factors may constrain the flexibility of firing rate interactions among M1 neurons.